There have been numerous attempts in the prior art of which I am aware to provide devices of the above mentioned type for presenting displays which when illuminated are sunlight readable and when not illuminated are not visible, but to my knowledge, none of these devices have proved to be entirely satisfactory.
An indicia display is herein defined to be sunlight readable if in a light ambient of 10,000 foot-candles (direct sunlight):
1. The contrast ratio of the lighted indicia to the background is 0.6 minimum, PA1 2. the contrast ratio of the lighted indicia to unlighted indicia is 0.6 minimum, and PA1 3. the contrast ratio of unlighted indicia to background is 0.05 (absolute value) maximum.
The contrast ratio is determined as defined in MIL-L-27160 as follows: EQU C=(C.sub.1 -C.sub.2)/C.sub.2
where C is the contrast ratio, and, for requirement (1) above C.sub.1 is the legend intensity of the lighted indicia and C.sub.2 is the background intensity; for requirement (2) above C.sub.1 is the legend intensity of the lighted indicia and C.sub.2 is the legend intensity of the unlighted indicia; for requirement (3) above C.sub.1 is the legend intensity of the unlighted indicia and C.sub.2 is the background intensity.
An indicia display device which is sunlight readable as that term is defined above will meet or exceed the requirements set forth by MIL-STD-411D, paragraph 5.1.
It is to be understood that the term "indicia display devices" when used herein, refers only to indicia display devices having the characteristics hereinabove discussed, and that the term "indicia display" when used herein refers only to an indicia display that is presented by an "indicia display device". It is also to be understood that an indicia display may be sunlight readable as defined above but not completely invisible (in direct sunlight) when not illuminated.
In an application where indicia display devices are in a high ambient light environment such as direct sunlight, the above defined sunlight readable capability, together with the capability of being invisible when not illuminated, is highly desirable. A high ambient light environment may be generally defined as one where the ambient light levels are within the range of 8,000 to 13,000 foot-candles. In some applications, as for example, aircraft cockpit instrumentation display and particularly in some military aircraft, such sunlight readable capability together with the capability of being invisible when not illuminated, can be critical.
Accordingly, it is the general object of the invention to provide improved indicia display devices which have a sunlight readable capability together with the capability of being invisible when not illuminated.
Another object of the invention is to provide indicia display devices which are sunlight readable.
Another object of the invention is to provide indicia display devices which are invisible in a 10,000 foot-candle light ambient (in direct sunlight) when not illuminated.
Another object of the invention is to provide indicia display devices that are sunlight readable and are invisible when not illuminated, and are incorporated in a pushbutton switch.
For a further understanding of the invention and further objects, features, and advantages thereof reference may now be had to the following description, taken in conjunction with the accompanying drawings.